Manta rays are diving deeper than we thought - but not for food

For a long time, scientists thought oceanic manta rays mostly stayed near the surface. These gentle giants are often spotted gliding through warm, coastal waters, sometimes in groups, sometimes alone.

However, new research shows that manta rays can plunge over 4,000 feet deep into the ocean. And they’re not going down there for food.

This behavior flips a lot of assumptions on their head. Most animals that dive that deep – like certain sharks or tuna – are hunting. But manta rays seem to have other reasons.

Researchers now believe that these deep dives might help them navigate across the open ocean.

Deep dives by manta rays

Oceanic manta rays are the largest species of ray on the planet. Some can grow over 20 feet wide. They’re known for long-distance travel and curious behavior, but they haven’t been known as deep divers – until now.

Over a ten-year study, researchers tagged 24 manta rays in three parts of the world: northern Peru, eastern Indonesia, and northern New Zealand. Using special tracking devices, the experts recorded over 2,700 days of movement data.

Eight of the devices were recovered, sending back high-frequency data every 15 seconds. The rest transmitted summary data by satellite. The researchers had to physically search the ocean to collect the floating devices, which wasn’t easy.

“It is quite a challenging task, trying to spot a small gray floating object with a short antenna bobbing around in the waves with other flotsam and jetsam,” said Dr. Calvin Beale, who led the study.

What the team found was unexpected. On 79 days, the tagged manta rays dove below 1,600 feet. On one dive, a manta reached 4,100 feet – over three-quarters of a mile straight down. Most of these deep dives happened off the coast of New Zealand.

Looking for clues in deep water

In New Zealand, the ocean floor drops off fast. Once the mantas left the continental shelf, they often dove deep the very next day. But they didn’t stay down long.

The dives followed a stepped pattern – like going down a staircase – and they didn’t spend much time at the deepest point.

That pattern tells scientists the rays aren’t hunting or avoiding predators. Most deep-diving predators spend longer at the bottom, where they look for prey. These manta rays didn’t do that. So why dive so deep?

Diving deep for mental maps

Researchers think the mantas are gathering environmental information. Deep in the ocean, conditions are more stable than at the surface.

The rays might be picking up cues like magnetic field changes, water temperature, or oxygen levels.

“By diving down and ‘sampling’ these signals, they could build a mental map that helps them navigate across vast, featureless stretches of open ocean,” said Dr. Beale, who completed his PhD at Murdoch University.

After a deep dive, mantas usually returned to the surface in stages, then hung out near the top for a while.

Many of these dives were followed by long-distance travel – some rays moved over 125 miles in the next few days. That supports the idea that the dives help them figure out where to go next.

Different oceans, different behavior

Not all the tagged manta rays behaved the same. In Peru and Indonesia, the rays didn’t dive as deep. This might be because they live in shallower areas and don’t need the same information to navigate.

In Indonesia’s Raja Ampat region, for example, the sea is mostly shallow. There are only a few deep-water channels. The mantas there may not need to go deep to orient themselves.

In New Zealand, by contrast, deep water is everywhere offshore. That might be why the mantas there dive so far down.

“Understanding the nature and function of deep dives helps explain how animals cross vast, seemingly featureless oceans and connect ecosystems thousands of kilometers apart,” noted Dr. Beale.

Why this matters beyond manta rays

This kind of research helps scientists understand how migratory species survive and move.

“Our study highlights how dependent migratory species are on both coastal and offshore habitats, stressing the need for international cooperation in their conservation,” said Dr. Beale.

“It also reminds us that the deep ocean – which regulates Earth’s climate and underpins global fisheries – remains poorly understood but vitally important.”

The team says more work needs to be done. This study used a relatively small number of tags and didn’t track the mantas continuously. Future research will need larger datasets to confirm why these animals are diving so deep.

Still, the message is clear: there’s a lot more going on beneath the surface than we thought.

The full study was published in the journal Frontiers in Marine Science.

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